1. Field of the Invention
This invention generally relates to systems that test integrated circuits and the like, and specifically relates to systems for measuring an integrated circuit's response to pulses generated by transmission line pulsers.
2. Description of the Related Art
It is useful to measure the ways integrated circuits (“ICs”) respond to short high power pulses when evaluating their electrostatic discharge (“ESD”) protection. The Transmission Line Pulse (“TLP”) techniques described in the ESD Association Standard Practice ANSI/ESD SP5.5.1-2004, as well as in U.S. Pat. Nos. 5,519,327 and 6,429,674, are often used to make such measurements. Specifically, these techniques are designed to measure the ways ICs respond to current and voltage pulses that are delivered to them by a pulse generator or pulser. The TLP pulser delivers an initial, or incident, TLP pulse through a constant impedance cable to a selected terminal of an IC or other device under test (“DUT”). When the incident TLP pulse reaches the DUT, it is partly reflected by the DUT and new current and voltage pulse waveforms result. The reflected pulse overlaps with the incident pulse as it travels back up the constant impedance cable in the opposite direction, toward the pulser. The relative amplitudes of these incident and reflected pulses are determined by the dynamic impedance of the DUT. The study of a DUT's response to short high power pulses is a common goal of ESD studies. The constant impedance cable is designed in a manner known in the art to avoid significant pulse distortions, so that the reflected pulse may be accurately measured and the dynamic impedance of the DUT may be calculated by comparing the ratio of the incident and reflected pulses.
Prior art circuits for measuring the incident and reflected current and/or voltage pulse waveforms typically use current and/or voltage oscilloscope probes. These probes are positioned in the constant impedance cable at a selected insertion point where the incident and reflected pulses are expected to overlap, which means that the probes need to be positioned only a small distance from the DUT terminal. This approach is particularly useful because when the probes are able to measure the voltage and/or current levels of incident and reflected pulses at a point where the waveforms overlap, it enables measurements to be made that approximate the actual current and/or voltage levels that appear where the cable connects to the DUT terminal, which is the desired measurement point to fully determine the DUT's response. Additionally, there are also signal measurement techniques well known in the art that improve the accuracy of such measurements but they can only be applied when the pulses overlap.
It is known in the art that the actual pulse at the DUT terminal under test is the sum of the incident and reflected pulses, and prior art circuits and devices are not designed to take an actual measurement of these waveforms. This is because prior art circuits measure the waveforms' overlap when the waveforms are slightly displaced in time relative to each other. The waveforms are displaced at the measurement point because prior art designs place their oscilloscope probes a small distance from the DUT, as mentioned above, while the incident and reflected pulses only perfectly overlap at the exact point where the constant impedance cable connects to the DUT.
In other words, there will always be a time delay between the incident and reflected pulses when there is any length of cable between the oscilloscope probe and the DUT, and this is a characteristic of prior art devices. The time skew can be very short, but for some TLP measurements a delay as short as one nanosecond will cause measurement errors. This is especially true for cases where the total pulse length is on the order of a few nanoseconds, the skew becomes a significant part of the waveform.
DUT response to a TLP pulse can be divided into two time regions, the initial transient response and the steady state response after the transient has dissipated. Prior art TLP systems measure only the steady state response because the overlapped waveforms do not show the overlap of the transient response. When measuring the transient response of the DUT, the first nanosecond is often the most important waveform measurement. The actual DUT waveform thus cannot be recorded using the way in which prior art systems place oscilloscope probes in the constant impedance cable.
TLP waveforms are typically recorded with single-shot high-speed digital oscilloscopes. Computers then use the digitized waveform information collected by these oscilloscopes to determine steady state pulse time regions and to calculate the current and/or voltage levels at the DUT by averaging the data in those regions. Currently available oscilloscopes have dynamic ranges limited by their 8-bit analog-to-digital converters to 256 voltage levels and have typical noise levels of four or more least significant bits (limiting signal-to-noise ratios to <64:1). Data averaging improves this limited signal-to-noise ratio, but the resulting signal-to-noise ratio may be inadequate for some applications. Techniques known in the art have been developed to optimize oscilloscope measurements under computer control whereby the oscilloscope input amplifier gain is increased and oscilloscope offset adjustments made to shift the waveform, to zoom in and record the desired measurement region of the waveform.
With overlapped incident and reflected pulses, known prior art techniques have controlled oscilloscope gain and offset to improve the signal-to-noise ratio by ten fold. However, when an oscilloscope's digitization of DUT waveform measurements do not have an overlapped area, or when such overlapped area is small compared to the pulse width, these techniques may not be effective to improve the signal-to-noise ratio. Importantly, when TLP pulse widths are less than 10 nanoseconds (commonly termed Very Fast TLP or “VF-TLP”), very little, if any, overlap of incident and reflected pulses is possible at the measurement point in the constant impedance cable. Without overlap, the incident and reflected pulses are separately recorded and a computer calculates a mathematically generated estimate of the DUT's current and/or voltage waveforms.
The ESD Association Standard Practice ANSI/ESD SP5.5.1-2004 document describes several configurations of TLP. The most commonly used is the Time Domain Refection (“TDR”) configuration. Most TLP systems produce 100 nanosecond-wide pulses. These systems employ oscilloscope measurement probes that usually capture the pulse signals where the incident and reflected pulses overlap in the constant impedance cable. This may be called “TDR-O,” which stands for TDR with overlapped pulses. In contrast, there are TLP systems that measure “TDR-S,” which is TDR using separated pulses where the constant impedance cable is long enough to hold the entire pulse length between the measurement point and the DUT. An advantage of measuring TDR-O is that the oscilloscope control system can optimize the vertical gain, adapting to the signal level of the best use of the oscilloscope's high-speed digitizer's dynamic range.
As previously mentioned, TLP using pulse widths of 10 nanoseconds or less is often called Very Fast TLP. Due to physical constraints of VF-TLP systems, the current and voltage measurement probes can not be placed close enough to the DUT terminal in the constant impedance cable to allow significant overlap of the incident and reflected pulses as required to make a useful TDR-O measurement. Therefore, TDR-S is the most commonly used configuration for VF-TLP. Unfortunately, oscilloscope gain adaptive gain control is very limited with TDR-S compared with TDR-O. What is needed is a circuit that converts TDR-S signals to TDR-O type signals and thereby resolve this measurement weakness.
A drawback of prior art techniques for measuring TDR-O signals is that, although the incident pulse and the reflected pulse overlap, but they are never fully overlapped. As the measurement probes cannot be placed exactly at the point where the transmission cable connects to the DUT terminal, there is always a finite time required for the incident pulse to travel from the measurement point to the DUT terminal. This same amount of time is required for a reflected pulse to travel from the DUT terminal to the measurement probe location. During this down and back travel time, the measurement probe records the incident pulse without the reflected pulse. Then the overlap period is measured, and finally the period where the reflected pulse does not overlap the incident pulse is recorded. Because the overlap is imperfect and the pulses are not perfectly rectangular, or flat topped, the actual DUT terminal current and/or voltage waveforms cannot be directly measured from the recorded signals. Information about the time-varying dynamic transient response of the DUT cannot be clearly established, either. A purpose of the present invention is to obtain fully overlapped pulses in order to provide recording of the undistorted DUT waveforms. These recordings provide both transient and steady state DUT response information.
Therefore, a need exits in the art of TLP systems that test the ESD protection of ICs to improve their measurement capabilities by producing a true replica of the DUT electrical signals as they exist at the DUT terminal under test, thereby enabling more accurate measurements of DUT responses to TLP pulses, including measurements of transient responses.